TERRESTRIAL DISTRIBUTION OF TEMPERATURE 71

that much snow accumulates; others are short and open. One year may
be warm and wet, and the crops abundant; but the next year may be so
dry that the farmers can scarcely raise enough to make a living. In
tropical and polar regions there are marked differences between different
years, although the variability is not so great as in the dry continental
interiors.

Climatic variations last through long periods as well as short. About
twenty-five or thirty thousand years ago occurred the last climax of
what is known as the glacial period. Ice many hundred feet thick covered
northwestern Europe and most of North America north of the Missouri
and Ohio rivers. Since that time the climate has changed so that most
of the ice has melted and some of the places which it covered have be-
come the most progressive parts of the world.

B. A SIMPLIFIED ROTATING GLOBE

Rotation as a Controlling Climatic Factor

The first step in understanding climate is to know why it differs from
place to place. These differences depend on four factors: (1) the earth's
rotation, (2)'the revolution of the earth and the inclination of its axis,
(3) the distribution of land and water, and (4) the relief of the lands.
In this chapter and the next we shall study each of these four factors
separately and consider its effect upon temperature, pressure, winds,
and rainfall.

Terrestrial Distribution of Temperature

The fact that the sun shines on a globe which rotates on an axis
causes the area where the temperature is high to form a belt surrounding
the earth completely where the sun's rays strike vertically. It also causes
belts of lower temperature to encircle the earth at higher latitudes. This
seems to be such a matter of course that we forget that it is possible only
on a rotating globe. The rapidity of the rotation, once in twenty-four
hours, prevents the temperature, winds, and rainfall from becoming so
extreme that life is impossible. If the earth did not rotate, a single
day and night would last a year. At any given place the movement of
the,earth in its annual orbit would at some time during the year bring
the sun above the horizon. Then for three months, as the sun slowly
climbed higher, the air would grow warmer and then hotter, until the
temperature became unendurable. Three months later, the sun would
set. Then would come a night of six months as cold and benumbing
as the day had been scorching and enervating. At all times there would